You can just cast it to Comparable. You're still casting, but you don't have to cast to the specific concrete type, so it should simplify your code.
This should be intuitive when you think about it. You give the method some object, about which the method knows little. But you happen to know that, whatever the object is, it implements Comparable. So by casting it as such, you're telling the method that.
That's just an anthropomorphic analogy but maybe it helps.